[Paper Review] NbReSi: A Noncentrosymetric Superconductor with Large Upper Critical Field

Hang Su, Tian Shang+7 more|arXiv (Cornell University)|Nov 11, 2021

Rare-earth and actinide compoundsPhysics and Astronomy65 references22 citations

TL;DR

This study reports the discovery of bulk superconductivity in the noncentrosymmetric compound NbReSi with a transition temperature Tc = 6.5 K. Electrical, magnetic, and heat-capacity measurements reveal a fully gapped superconducting state with a large upper critical field of µ0Hc2(0) ≈ 12.6 T, approaching the Pauli limit, while band-structure calculations confirm significant spin-orbit coupling-induced band splitting.

ABSTRACT

We report the discovery of superconductivity in noncentrosymmetric NbReSi, which crystallizes in a hexagonal ZrNiAl-type crystal structure with space group $P\bar{6}2m$ (No.~189). Bulk superconductivity, with $T_c$ = 6.5 K was characterized via electrical-resistivity, magnetization, and heat-capacity measurements. The low-temperature electronic specific heat suggests a fully gapped superconducting state in NbReSi, while a large upper critical field of $\mu_0H_\mathrm{c2}(0)$ $\sim$ 12.6 T is obtained, which is comparable to the weak-coupling Pauli limit. The electronic band-structure calculations show that the density of states at the Fermi level are dominated by Re and Nb $d$-orbitals, with a sizeable band splitting induced by the antisymmetric spin-orbit coupling. NbReSi represents another candidate material for revealing the puzzle of time-reversal symmetry breaking observed in some Re-based superconductors and its relation to the lack of inversion symmetry.

Motivation & Objective

• To investigate the superconducting properties of the noncentrosymmetric NbReSi, a ZrNiAl-type compound with no prior characterization.
• To determine whether NbReSi exhibits unconventional superconductivity, including fully gapped or nodal behavior.
• To assess the role of antisymmetric spin-orbit coupling (ASOC) in inducing large upper critical fields and potential time-reversal symmetry breaking.
• To clarify the interplay between noncentrosymmetry, ASOC, and superconducting pairing in Re-based materials.

Proposed method

• Polycrystalline NbReSi samples were synthesized via arc melting and characterized using powder X-ray diffraction (XRD) to confirm the ZrNiAl-type structure with space group P¯62m.
• Electrical resistivity, magnetization, and heat-capacity measurements were performed using a PPMS and MPMS system to probe superconducting transitions and critical fields.
• Upper critical field Hc2 was extracted from field-dependent magnetization isotherms, with Hc1 determined from the onset of nonlinear magnetization response.
• Electronic band-structure calculations were performed using density-functional theory (DFT) with the PBE functional in VASP to analyze the Fermi surface and spin-orbit coupling effects.
• The Ginzburg-Landau theory was applied to estimate the coherence length and upper critical field, comparing results with the Pauli limit.
• Muon-spin relaxation (µSR) was referenced in context to previous studies on related Re-based superconductors to frame the significance of time-reversal symmetry breaking.

Experimental results

Research questions

• RQ1 Does NbReSi exhibit superconductivity, and what is its transition temperature Tc?
• RQ2 Is the superconducting state in NbReSi fully gapped or nodal, as indicated by electronic specific heat?
• RQ3 What is the magnitude of the upper critical field Hc2, and does it exceed the Pauli limit?
• RQ4 To what extent is antisymmetric spin-orbit coupling (ASOC) responsible for the observed electronic band splitting and superconducting properties?
• RQ5 Does the lack of inversion symmetry in NbReSi lead to time-reversal symmetry breaking, as seen in other Re-based superconductors?

Key findings

• NbReSi exhibits a bulk superconducting transition at Tc = 6.5 K, confirmed by zero resistivity and a sharp drop in magnetic susceptibility.
• The electronic specific heat data below Tc are consistent with a fully gapped superconducting state, with no evidence of nodes.
• The upper critical field µ0Hc2(0) reaches 12.6 T, approaching the weak-coupling Pauli limit, indicating strong spin-orbit coupling effects.
• Band-structure calculations show that the Fermi surface is dominated by Re and Nb d-orbitals, with significant splitting due to antisymmetric spin-orbit coupling.
• The crystal structure lacks inversion symmetry, with Nb and Re atoms occupying noncentrosymmetric sites, supporting the potential for unconventional pairing.
• The large Hc2 value in a noncentrosymmetric system suggests a possible link to spin-triplet pairing or time-reversal symmetry breaking, though no direct evidence for the latter is found in this study.